More power, more mileage

As electrical loads rise, traditional 14V
systems will soon come up short. A 42V electrical architecture will help
meet new demands for power.

The growing appetite for automotive electrical power at first glance seems at odds with fuel economy. "Everything that consumes electric power potentially hurts fuel economy," says Phil Chapekis, manger of higher power vehicles for Visteon Automotive (Troy, MI). And electrical loads will be sure to rise as automakers adopt 42V electrical systems. Even a doubling of today's 2kW maximum load would increase fuel consumption as much as 3 liters/100 km, according to Beda-Helmut Bolzenius, president of Bosch. One solution: more efficient electric machines.

With more efficient power generation, the auto industry's impending shift from 12V to 42V electrical systems could be a real energy saver. The National Research Council estimates that 42V electrical systems can produce a 1-2% reduction in fuel consumption just by reducing the electrical losses associated with various on-board electronics. Even bigger gains could come through a transition from wasteful mechanical systems to efficient electrical ones.

Fuel saving systems. For example, the extra jolt of power offered by a 42V system makes camless valve actuation a real possibility. Fuel economy could improve as much as 15% compared to a fixed-timing, four-valve engine, according to the National Research Council. In the past, the development of electromagnetic valve trains had been held back by the expected power requirements. "It will take between 2 and 2.5 kW of power for camless valve actuation," explains Chapekis. Other new fuel-saving electrical systems: steering, thermal management, and various engine-accessory functions. Chapekis says these could add up to another 5%—a real bonus since fuel economy testing ignores energy from steering or "parasitic" engine accessories.

Another large contribution to fuel economy: the integrated starter generators (ISG) that most 42V vehicles will use. "For large vehicles, present electrical systems won't meet cranking requirements," says Iftikahr Khan, manager of vehicle electrical systems for Delphi Automotive (Troy, MI). Used in place of traditional alternators and starter motors, ISGs offer three fuel-saving strategies: Start-stop operation allows the engine to shut down at idle and quickly switch on when it's time to move; Regenerative braking systems capture and store energy from deceleration; And launch assist provides torque to overcome standing inertia.

The National Research Council says that start-stop capabilities alone would result in 4-7% improvement in fuel economy. Regenerative braking and launch assist could add another 5-10%, says the Council. These estimates may even be a little conservative. "Twelve percent seems like a sure bet," Kahn says.

The picture for engines under two liters may be different. "(The ISG) price is too high for small vehicles," Khan says. Small car solutions will include 14V systems incorporating the crank function into the alternator. The result: Kahn estimates a 6-7% increase in fuel economy.

Better alternators needed. Power-generation efficiency is key. Though cost-effective, the Lendell alternators commonly used today have average efficiencies, depending on load, of about 55%, says Khan. Optimization of conventional alternators has bumped up those efficiencies. Bosch, for example, claims its latest alternator reaches 72% efficiency at 14V thanks to proprietary winding technology that reduces stator losses. "This increase would have been impossible a few years ago," Bolzenius says.

Yet automotive system suppliers estimate that 42V electric machines need to be at least 80% efficient to reconcile skyrocketing power and tightening fuel-consumption requirements. And current fuel economy standards don't capture the impact of electric accessories that will likely proliferate on 42V vehicles.

Several technologies could help: permanent magnet, induction, and permanent-magnet-enhanced machines. Expect technologies with the highest power densities to win. They will limit size and weight increases in power generation and storage systems, Chapekis explains. Cost will, as always, also play a role. Chapekis notes that the newer electrical machines are based on more expensive materials. They also require more sophisticated power electronics and controls—in part because higher-voltage vehicles will likely include enhanced controls that match electrical power to demand. Suppliers say it's too soon to know the extra costs until manufacturing volumes ramp up fully.

Still, says Kahn, "It's not a matter of choice. We must go to machines with higher efficiencies."

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